Interferometry superposes (interferes) two or more (electro-magnetic) waves, to detect differences between the waves, i.e., their interfering characteristics. The most common interferometer is the Michelson type of interferometer. The basic components of the Michelson interferometer are a light source, a detector, two mirrors and one semitransparent mirror, often called a beam splitter.
A simple interferometer counts the “fringes” where the intensity of the output wave runs in a complete cycle from maximum (where the waves constructively interfere) to a minimum (complete destructive interference) and back to a maximum again. These intensities are sine waves. Because of this single output, a simple interferometer can detect a change of differential distance in the path of the wave, but not the direction of this change.
When a phasing arrangement (such as a quarter-wave retarder) is used to generate a pair of paths (usually by having two beams, one in each polarization, operating through the same set of lenses and mirrors) one polarization path is delayed by the quarter-wave retarder so that the resulting sine wave intensity is delayed 90 degrees from the unretarded path. This gives a pair of outputs for the interferometer. These outputs are in quadrature, and like a standard quadrature encoder, can detect both change in distance and the direction of the change.
Unfortunately, the resolution of an interferometer of this form is one quarter of a wavelength of path difference; in the case of a typical interferometer using a helium-neon (HeNe) laser that wavelength is fixed by the physics of helium and neon at 632.818 nanometers and cannot be changed. Thus, the minimum distance directly encoded by the quadrature output of the interferometer is ¼ of the 632.818 wavelength, or 158.20 nanometers. This quarter-wave distance is sometimes called the “native resolution” of the interferometer.
The current state of the art provides further gains in resolution by intensity interpolation; that is, the relative powers of the undelayed and delayed paths are measured by photodiodes, the photodiode current is amplified and converted to digital values by A/D converters, and software is used to map this pair of relative power values as sine and cosine values onto a unit circle. As each of the arcsine or arccosine values yields two possible angles on the unit circle, the two values will produce a total of three simultaneous possible angles on the unit circle, but only one value (the duplicated value) is correct. This unit circle angle divided by 2π is the fraction of a wavelength that the beam path has changed from the interferometer zero point and in state of the art interferometers can often be as small as 1 part per 100 of a wavelength, or 25 times the native resolution.
The disadvantages of the conventional methods are that the two parts of the readout are of different types: the number of quarter-wavelengths is read out by quadrature encoding in real time, while the fraction of the wavelength is the result of a software interpolation which is delayed by some amount of time. Merging these values together causes yet more delay, so the overall interferometer system does not report on the current position, but on the position at some point in the past, typically at least 50 microseconds in the past. This corresponds to an update rate of about 20 K Hz and by the Hamming theorem, no motion of higher than one half that rate (10 K Hz) can be detected by such an interferometer.
Prior art interferometers are severely limited in their speed, typically in the range of about 20 K Hz, and often much less. Many marketed interferometers have update rates as low as 10 to 20 Hz. Conventional interferometers are usually fixed, and configured for a particular operating condition. Changing the operation of the interferometer in real-time is impossible in most cases.
It is desired to increase the speed of interferometers at least a thousand-fold or three orders of magnitude (103) to well into the Mega-Hertz range. In fact, it desired to increase the speed so that the speed of the interferometer is mainly limited by the speed in which the intensities of optical signals can be converted to digital signals. Thus, as the speed of the conversion increase, so will the speed of the interferometers that use the invention. It is also desired to dynamically adapt the interferometer to its operating characteristics and environment.
The concept of interferometry as an experimental science dates back to Albert Michelson, who was awarded the Nobel Prize in 1907. R. C. Moore in U.S. Pat. No. 4,583,856 uses a computer to calculate the sub-fringe resolution in a laser interferometer. Distance measuring interferometers are commercially available. A top-of-the-line laser interferometer from 4D Technology operates with a sample time of 30 microseconds and updating within one frame time, that is, a 20 K Hz final update rate. The Canon micro-laser interferometer limits the working range to +/−50 microns, this micro interferometer can update at 100 K Hz.
One goal of this invention is to radically increase the speed of the interferometric measurement well up into the M Hz range or greater.
Another goal of this invention is to merge the output formats of the interferometer such that any device reading the interferometer measurements can read the output of the interferometer as any format of value desired, whether that is as a Gray code, as an integer, as quadrature-encoded pulses, or whatever other arbitrary format may be useful. Thus, it is desired to make the interferometer dynamically configurable to any desired data format, perhaps while the device is operating, or in response to its operating characteristics and the environment in which it operates.